The present invention relates in general to the construction of auxiliary cooled nozzles, and in particular to a new and useful method of constructing a hollow body of revolution on which is particularly useful as a rocket nozzle.
A rocket nozzle may be approximated by a thin-walled shell of revolution having the shape of a conic section (ellipse, parabola or hyperbola). The shape directly effects the performance of the rocket engine by directing the engine exhaust gases. Nozzles are either uncooled or auxiliary cooled to survive the operating temperatures of the engine, where the design is dependent upon the material used. In general, uncooled rocket nozzles are made from refractory alloys such as columbium. Auxiliary cooled nozzles are produced from lower cost, lower melting point materials such as nickel/cobalt-base alloys. Survival of these nozzles is dependent upon the presence of auxiliary coolant, which may include liquid hydrogen fuel or engine exhaust gases.
Auxiliary cooling is provided by attaching individual lengths of tubing on the inside surface of the nozzle. These tubes must be positioned parallel to the axis of rotation for the nozzle so as not to create turbulence in the engine exhaust gases. This would lower performance and create potential instabilities. The tubes are attached to the surface of the nozzle by furnace brazing, where brazing foil is attached to the nozzle surface prior to inserting the tubes. This operation generally requires a number of subsequent brazing operations to achieve complete bonding at the interface. These brazing operations are performed in an atmospherically controlled furnace to prevent oxidation, and therefore are quite costly and labor intensive. Depending on the size of the given nozzle, the number of tubes required can exceed several hundred.
The geometry of the each tube is such that it must conform to the conical or parabolic shape of the nozzle, thus requiring custom forming. Additionally, the entire inside surface of the nozzle must be covered to prevent "hot spots" which could result in premature failure of the nozzle. Each nozzle has an aspect ratio from the aft or large outlet end of the nozzle to the forward or small inlet end of the nozzle. Typical aspect ratios range from 2:1 to 3:1. Since there are no intermediate manifolding techniques for the tubes, this requires the same number of tubes at both ends of the nozzle. Therefore, to completely cover the nozzle surface, requires each tube to have a custom shape, which must change along its length, and is directly related to the aspect ratio of the nozzle. This can be in the form of increasing diameter (forward to aft) at a rate equal to the aspect ratio, or by changing the shape of the tube (e.g. making it oval) as it approaches the forward end of the nozzle. The first method results in a tube having an increasing flow area along the length of the nozzle, which can be undesirable for efficient cooling. The second approach is more ideal, where a constant perimeter is maintained by altering the tube geometry. In either case, the tubing is quite expensive.
At the forward end of the nozzle, the tubes are attached to a manifold to allow the auxiliary coolant to flow through the tube passages. The aft end of the channel can be designed to recirculate the coolant back up the nozzle wall, or to vent the coolant to either the atmosphere or the engine exhaust.